1. Field of the Invention
The present invention relates to a method for producing a cylindrical bushing with hydraulic damping and, more particularly, to a method for efficiently producing a cylindrical bushing having excellent durability.
2. Description of the Prior Art
A cylindrical bushing with hydraulic damping has an outer cylinder, an inner cylinder disposed within the outer cylinder, and a vibration-damping rubber block interposed between the outer cylinder and the inner cylinder. Such a cylindrical bushing achieves an effective vibration-damping operation with a simple construction, frequently used in an engine mount for a motor vehicle. Recently, there has been proposed a new type of cylindrical bushing of which the rubber block contains therewith liquid chambers, each enclosing a damping liquid. With this bushing, vibrations, particularly those of large amplitudes are effectively damped by the operation of the damping liquid.
The cylindrical bushing having the liquid chambers has been produced by forming a vibration-damping rubber block integrally with a cylindrical internal ring, reducing the diameter of the internal ring in order to cancel the tensile stress which has been generated within the rubber block during the formation thereof, and press-fitting the internal ring into the outer cylinder.
With this conventional producing method, wall portions of the rubber block, which define the liquid chamber are also compressed in the reducing step. When a static load of an engine of a motor vehicle is applied to the inner cylinder, it displaces with respect to the outer cylinder. This causes the compressed wall portions to be further compressed. Accordingly, the durability of the wall portions of the rubber block decreases.
The cylindrical bushing is required to provide a secure liquid-tight seal between the outer cylinder and the internal ring. To this end, a rubber seal layer conventionally has been formed therebetween. However, upon reducing of the internal ring, this rubber seal layer is compressed and wrinkles are likely to be formed therein. This causes the decrease in sealing properties of the rubber seal layer.
In order to overcome the above-described disadvantage, conventionally, the rubber seal layer has been formed along the inner periphery of the outer cylinder, and then the internal ring of which the diameter is reduced has been press-fitted into the outer cylinder. This method is unfavorable because of low production efficiency.